Lithium-ion batteries are rechargeable batteries that have a wide variety of uses. In general, lithium ions move from a negative electrode to a positive electrode during discharge, or during use of the battery, and from the positive electrode to the negative electrode when the battery is being charged. Lithium-ion batteries are common in electronics, space applications, electric vehicles, and military applications due to their high energy-to-weight ratio and slow loss of charge when the battery is not being used. Lithium-ion batteries are significantly lighter than equivalents in other chemistries such as, for example, lead-acid, nickel-metal hydride, and nickel cadmium.
Silicon is an attractive anode material for lithium-ion batteries because of its low discharge potential relative to alternative anode materials. In addition, silicon has the highest known theoretical charge capacity of 4,200 mAh/g. Despite its capacity advantage over existing graphite anodes (327 mAh/g) and various nitride and oxide materials, silicon anodes have limited applications because silicon's volume changes by about 300% upon insertion and extraction of lithium. This volume change results in pulverization of the silicon anode as well as capacity fading. Current approaches addressing the cycling issues of silicon anodes include pure silicon micro- and nano-scale powder anodes, silicon dispersed in an inactive matrix, silicon dispersed in an active matrix, silicon anodes with different binders, and silicon thin films.
Using a pure form of silicon such as silicon bulk films and micrometer sized particles as anodes in lithium batteries shows capacity fading and short cycle life due to pulverization and loss of electrical contact between the active material and the current collector. An efficient approach showing promising results is to stabilize silicon by forming alloys. One of the major limitations and issues, however, is the low concentration of silicon in the alloys. The inactive alloying metals that are added to the system can make overall system capacity suffer. Less stabilizing metals result in an increase in silicon random crystalline domains, which can crack upon lithiation and de-lithiation.
To achieve a 1:1 or higher silicon ratio in the composite, thick polycrystalline or amorphous silicon domains can be formed by conventional processes, including chemical vapor deposition (“CVD”). These domains are known for their poor cycle life due to the lack of an electrical conduction path.